Despite the fact that by virtue of available modem video-assisted endoscopic instrumentation significant progress has been achieved in urological invasive treatment in general and in the extracting of foreign objects in particular, the evacuation of such objects like calculi from a surgical patient's body still remains a challenge for a surgeon.
In the course of complicated and time-consuming surgical treatment the removal of calculi of different sizes and characteristics from various sites along the urinary tract and from various locations within the body, (e.g. removal of gallstones and kidney stones) becomes essential. This challenge has resulted in the development of a variety of surgical tools for stone retrieval without the need for major surgery. The calculi retrieval tools, or so-called surgical extractors usually comprise a flexible tubular catheter formed as a tubular sheath adapted to penetrate along the body passages to reach the location of the object to be evacuated. A wire or cable is inserted within the catheter, which can be manipulated from the outside at the catheter's proximal end. The cable is connected to a basket deployed within the sheath at the catheter's distal end. The basket consists of flexible filaments made of either a memory shape material (e.g., stainless steel) or any other material capable of providing the basket with elasticity.
Depending on the manipulation, the basket may either retract inside the sheath to allow penetration of the catheter via passage or protract from the catheter. In the protracted position, the wires open due to the elasticity of their material and form a cage thus allowing the object to enter inside the basket through the open spaces left between its adjacent wires. Further retraction of the basket inside the sheath results in collapsing the cage and entrapping the object in the basket. Removal of the catheter will enable the whole device to be removed from the body organ together with the object immobilized within the basket.
It can be easily appreciated that the particular design of the basket-like element is crucial for entrapping and reliable retaining the object during evacuation.
Various attempts to devise the basket are known in the art. Examples of conventional baskets can be found in the web site www.bsci.com. According to the recent classification suggested by Boston Scientific Corporation, the currently employed baskets for stone retrieval can be broadly categorized into the following groups: (11) Flatwire baskets, (12) Helical baskets and (13) Multi-wire baskets.
An example of a flatwire basket is disclosed in U.S. Pat. No. 6,183,482. This basket comprises one or more legs to retrieve the calculi. At least one of the legs has an inner surface and an outer surface. The outer surface is curved to render the basket atraumatic. The inner surface can be flat such that the leg has a D-shaped cross-sectional configuration. It is reported in the literature that flatwire baskets minimize lateral basket wire movement and this facilitates and improves stone capture. Furthermore, flatwire baskets open reliably even in an undilated ureter. Flatwire baskets are available in a wide range of sizes and wire configurations. A method of manufacturing of such baskets is described, for example, in U.S. Pat. No. 5,658,296.
The intrinsic disadvantage of flatwire baskets is their unsuitability for capturing and immobilizing relatively small calculi, since the amount of wires in such baskets is limited to very few and the size of open space between the adjacent wires remains too large.
In an attempt to solve this problem and allow the capturing and retaining of small calculi, so-called helical baskets were devised. Examples of surgical extractors equipped with the helical baskets can be found in U.S. Pat. Nos. 3,496,330; 6,190,394; 4,347,846.
It is appreciated in the art that helical baskets incorporate strong, flexible wire construction in a spiraled shape design. The streamlined, spiraled shape is well suited for efficient, effective stone capture. As the basket is drawn back over the entrapped calculi, the configuration of the wires “sweeps” the stone into the basket.
U.S. Pat. No. 5,496,330 discloses a helical basket formed as a plurality of strands encased in a sheath and wrapped in a helical form. The displacement of a portion of the threads from the sheath causes their wide angular disposition to form a basket for retrieving the object. The threads include a plurality of individual filaments that are closely spaced through the length of the basket. By virtue of strands, formed as multiple, closely situated adjacent wires, the number of contact points with entrapped calculi is increased without requiring a concomitant increase in the size of the overlying sheath.
Nevertheless, one can appreciate that the above-mentioned advantage associated with the increased number of contact points may limit this basket to the treatment of small calculi and render this basket less suitable for the immobilization of relatively large calculi. The reason for this is the close angular disposition of the filaments that prevents the entry of large calculi in the limited open space between the adjacent filaments.
U.S. Pat. No. 6,190,394 discloses a medical retrieval basket formed as a plurality of flexible elements that are outwardly disposed to form a cage for entrapping objects therein. This basket was devised to enable the efficient entrapment of the objects and their reliable holding within the cage. To achieve this goal, the basket is manipulated by rotation and employs flexible elements having different sizes, different cross-sectional configuration and/or different spacing.
In one embodiment, the cage consists of non-twisted flexible elements as in flatwire basket design. In the other embodiment, the flexible elements are helically twisted. In both embodiments, the cage is symmetrical with respect to a plane drawn perpendicularly to the middle of the cage length.
It can be appreciated, however, that the above-mentioned intrinsic disadvantages associated with flatwire and helical baskets would be characteristic also to the basket disclosed in U.S. Pat. No. 6,190,394. The further disadvantage of the helical design is associated with the fact that parallel-directed helically shaped filaments are prone to entanglement.
U.S. Pat. No. 4,347,846 discloses a surgical extractor employing a cage or basket formed by steel wires and disposed in a helical path. Some of the wires follow a helix in clockwise direction, while other wires, in equal number, follow a helix in anti-clockwise direction. By virtue of this extractor, the reliability of retention of the body during evacuation is improved, since the body may enter within the extractor sheath when the cage is being retracted. This prevents the accidental escape of the body from the cage. Furthermore, by virtue of the opposite direction of the helically shaped filaments, any danger of their twisting and entanglement of the cage is avoided.
It should be emphasized that the general shape of the cage employed in the extractor described in U.S. Pat. No. 4,347,846 is very similar to the shape of helical basket disclosed in U.S. Pat. No. 6,190,394. This shape is also symmetrical to a plane, drawn perpendicularly to the middle of the basket's length. Therefore, the same above-mentioned intrinsic disadvantages are characteristic to this helical basket as well. Furthermore, since the filaments are not bound together and only have points of passive contact in which they overlap, it can be appreciated that the cage will neither have sufficient radial rigidity to prevent escape of the calculi between adjacent filaments, nor sufficient dilatation ability to provide enough room between the entrapped calculi and the tissue of the passage. Thus, either the loss of calculi during evacuation or damage to the adjacent tissue might occur during the treatment.
In an attempt to overcome the disadvantages associated with the entrapment and retention of calculi of various sizes, a multi-wire parachute basket having two sections have been devised. In the first section, the amount of filaments is small and they are in spaced relationship to enable the easy passage of the calculi through the spaces between the filaments. The second section is formed as a plurality of filaments, extending from the filaments of the first section and defining a multi-wire cage. Since in this cage the filaments are in close relationship, it is possible to ensure more complete engagement of the captured calculi when the basket is in a protracted position and a more reliable grip when the basket is being retracted within the sheath.
One early attempt to devise a basket in accordance with the parachute concept is described in U.S. Pat. No. 3,472,230. This patent discloses a retrieval basket made of four spring wires connected to a slide. An umbrella made of suitable flexible web material is connected to the distal ends of the wires so that upon deployment of the basket the edges of the umbrella form a scraper. Retracting the basket is associated with scraping the debris from a body passage and its retaining within the umbrella.
The other example of the retrieval parachute basket, provided with web umbrella, is presented in U.S. Pat. No. 4,790,812. The disadvantage of the parachute baskets provided with the web umbrella is associated with difficulty in manufacturing and with the relatively large size of the web umbrella in the compacted condition.
The further progress in the design of parachute baskets have been achieved by devising multi-wire parachute baskets (see, for example, S.U. Pat. No. 1036325; R.U. Pat. No. 2022528, U.S. Pat. No. 6,168,603).
S.U. Pat. No. 1036325 describes a surgical extractor with multi-wire parachute basket formed with a first section, which is proximal to the sheath and with a second section, which is distal thereto. The proximal section comprises two separate branches extending from the sheath in a V-shaped fashion. The branches comprise strings that are woven from the individual wire filaments. From distal ends of the branches emerge individual wire filaments, which are bent as elongated elliptical loops. The loops emerging from the branches meet in a common point at the distal end of the extractor. Additional wire filaments are provided longitudinally extending within the loops. These filaments divide the loops into supplemental branches, which together define the second section of the basket.
The disadvantage of this basket is associated with the fact that it still does not always ensures reliable retention of the calculi, since the amount of branches along the basket is invariant and their filaments are parallel and separate. Thus, small calculi might escape through the room between the branches of the second section.
A similar construction of the multi-wire parachute basket is disclosed in U.S. Pat. No. 6,168,603. In this US patent is described surgical extractor on whose distal end is arranged a plurality of wires defining a retrieval basket. Each wire comprises a first portion having an individual strand and a second portion comprising a plurality of filaments. When a retaining sheath of the extractor is retracted the wires formed of a shape memory material expand. Each strand assumes a spaced relationship to define a first section of the basket. A plurality of filaments define a second section of the basket. Widely spaced wires of the first section promote capturing the object and closely spaced wires of the second section enable retention of the captured object. It is worth mentioning that in one of the embodiments the second section is formed from filaments which are helically wound and angularly spaced.
R.U. Pat. No. 2022528 discloses a surgical extractor provided with a basket formed from at least two branches made of metallic strings. The main branches define the first section of the basket, which is intended for entering the calculus within the basket. Each branch forms at its distal end sub-branches, which are made of two main sub-branch and one auxiliary sub-branch. The sub branches define the second section of the basket. The sub-branches and the auxiliary branches have different thicknesses and different elasticities. By virtue of this basket, the reliability of entrapment and immobilization of the entrapped calculus is improved, since its construction allows for the variation of the amount and arrangement of the sub-branches.
However, the above mentioned multi-wire parachute baskets only partially solve the problem of reliable capturing, immobilization and holding of calculi, irrespective of their size. The reason for this is the size of the open space along the branches, which is equal to the length of filaments of the second section. Calculi whose size is shorter than this length can easily escape from the basket. Furthermore, in the above baskets the filaments of neighboring branches are either fully separate (as in S.U. Pat. No. 1036325) or only passively overlap (as in U.S. Pat. No. 6,168,603 and R.U. Pat. No. 2022528) and they are not entwined. By virtue of this provision the radial rigidity of the cage is very limited and might be not sufficient to prevent the possibility of the adjacent filaments being spread apart by the calculus during entrapment, and the calculus thus escaping. Insufficient radial rigidity is associated also with the possibility of damaging adjacent body tissues by the entrapped calculus protruding through the open space formed by the spread filaments. The insufficient radial rigidity deteriorates the dilatation ability of the whole basket and therefore contributes to the possibility of damage occurring to the nearby body tissues.